Recently, a DPF apparatus has been developed in order to purify PM (Particulate Matter) from exhaust gas of a diesel engine. Further, an LNT (Lean NOx Trap) catalyst has been developed in order to purify NOx contained in the exhaust gas. In a DPF system where an exhaust gas purification apparatus is connected to an exhaust pipe, when the purification efficiency of the purification apparatus is decreased, the combustion and removal (DPF forcible regeneration) of PM deposited on the DPF using high-temperature exhaust gas or the rich reduction of the LNT catalyst is performed by adding unburned fuel to the exhaust gas and then oxidizing and combusting the added unburned fuel with a DOC (Diesel Oxidation Catalyst) provided in the exhaust pipe.
An exhaust pipe injection has come into the spotlight as means of adding the unburned fuel to the exhaust gas in that EGR (Exhaust Gas Recirculation) control can be performed without occurrence of oil dilution in an engine cylinder even when fuel is added to hold down fuel consumption required for raising the temperature. The exhaust pipe injection is a method of adding the unburned fuel to the exhaust gas from an exhaust pipe injector provided in an exhaust pipe (for example, see Patent Documents 1 and 2). In addition, the exhaust pipe injection can perform an air-fuel ratio rich control of the exhaust gas regardless of engine combustion during the rich reduction of the LNT catalyst.
In the DPF system using the exhaust pipe injection, it is desirable to lower variation in flow rate of an exhaust injection amount from an exhaust pipe injector in order to stably perform the PM combustion and the rich reduction.